Advances in the Use of Stress Relaxation Data for Design and Life Assessment in Combustion Turbines

This paper describes developments in the use of the high-precision stress relaxation test (SRT) as a basis for accelerated creep strength evaluation. In tests lasting less than one day a creep rate range of about five decades may be covered. It is shown how the approach is especially useful for superalloys and their application in combustion turbines. Using creep rate data, derived from the SRT tests, for conventionally cast, directionally aligned and monocrystalline alloys, a new framework for design and life management is described. By parameterizing the results over a range of temperature it is possible to attain design stress levels in less than a week of testing. The test may be used to determine the effect of microstructural features such as grain size and grain orientation, thermal-mechanical exposure conditions, test specimen section size, and the effect of location within a component.